There is known a technology that, in a projector system having a personal computer (PC) and a projector for projecting and thereby displaying an image connected to each other via a Universal Serial Bus (USB) cable, the PC supplies the projector with image data, and then the projector projects and thereby displaying the image (see, e.g., Japanese Patent Publication No. 2004-88194).
Further, the applicant of the invention has put the projector system into practical use, in which the PC and the projector are connected to each other via the USB cable, the PC supplies the projector with image data and sound data, and the projector projects and thereby displaying the image, and at the same time outputs the sound from a speaker incorporated in the projector. More specifically, in the projector system, the PC transmits the digital data of the image to be displayed and of the sound to be output, to the projector via the USB cable in accordance with the process of an operating system and an application program by the central processing unit (CPU) of the PC itself. Then, the projector receives the digital signal, converts it into an image signal and a sound signal, and then performs the display by projection and the sound output from the speaker. Since in this projector system the conventional connection environment using a plurality of signal cables such as an RGB cable and an audio cable can be changed to the connection environment using a single USB cable, the burden of the complicated connection operation can be reduced, and at the same time, the image and the sound can be communicated between the devices as the digital data, and thus the scalability of the system can be enhanced.
In the image/sound supply technology described above, in order for reducing the processing load on each of the PC and the projector, and for reducing the amount of data transmitted between the devices, it is arranged that rectangular areas including portions with movement between the frame images are extracted from the image data, and then the data of the rectangular areas are transmitted. Further, in order for making it easy for the projector to synchronize the image and the sound with each other, it is arranged to transmit the data of the rectangular areas and the sound data alternately. In this case, in order for preventing the sound interruption from occurring in the reproduction of the sound in the projector, it is desirable for the PC to transmit a predetermined amount of sound data enough for preventing the lack of data in the sound processing in the projector at predetermined intervals. However, in the case in which the area with motion between the frames is large, and therefore, the amount of data of the rectangular area thus extracted becomes larger, it is caused that the time necessary for transmitting the data of the rectangular area exceeds the transmission intervals, and in this case, the sound interruption should be caused in the reproduction of the sound by the projector.
Here, it is conceivable that if the maximum amount of data of the rectangular area, which can be supplied at a time, is set to be equal to or smaller than the amount of data, which can be transmitted within one cycle of the sound data, and the PC controls the data of the rectangular area to be transmitted to be equal to or smaller than the maximum amount of data, the sound interruption in the projector can be prevented from occurring. However, since in the case in which the data of a plurality of rectangular areas with amounts of data different from each other are transmitted alternately with the sound data, there might be caused wait time between a period from when the data of the rectangular area has been transmitted to when the transmission of the sound data to be transmitted subsequently is started, the overall transmission efficiency might be degraded.